Firebase realtime database rules to edit only existing fields

Question

My database:

I want to set firebase rules where user can update existing tags value but not create new tags

Answer 1

If you only want the user to be able to update existing properties, and not create any new properties, that'd be:

{
  "rules": {
    "$uid": {
      ".write": "auth.uid === $uid"
      "$property": {
        ".validate": "data.exists()"
      }
    }
  }
}

So this rule validates that a property can only be written if there's already data for that property in the database.

---

If you only want the user to be able to write a name and address, and not any other properties, that'd be:

{
  "rules": {
    "$uid": {
      ".write": "auth.uid === $uid"
      "name": {
        ".validate": true
      },
      "address": {
        ".validate": true
      },
      "$other": {
        ".validate": false
      }
    }
  }
}

In the above rules we allow writing the named name and address properties, and reject any other properties with a wildcard capture rule.